Yu Hsiang Yeh

Vertical InGaN-based green-band solar cells operating under high solar concentration up to 300 suns

InGaN/GaN-based solar cells with vertical-conduction feature on silicon substrates were fabricated by wafer bonding technique. The vertical solar cells with a metal reflector sandwiched between the GaN-based epitaxial layers and the Si substrate could increase the effective thickness of the absorption layer. Given that the thermally resistive sapphire substrates were replaced by the Si substrate with high thermal conductivity, the solar cells did not show degradation in power conversion efficiency (PCE) even when the solar concentrations were increased to 300 suns. The open circuit voltage increased from 1.90 V to 2.15 V and the fill factor increased from 0.55 to 0.58 when the concentrations were increased from 1 sun to 300 suns. With the 300-sun illumination, the PCE was enhanced by approximately 33% compared with the 1-sun illumination.

Gallium nitride-based light-emitting diodes with embedded air voids grown on Ar-implanted AlN/sapphire substrate

GaN-based light-emitting diodes (LEDs) grown on sapphire with ex situ AlN nucleation layer prepared by radio-frequency sputtering were investigated. GaN-based epitaxial layers grown on the Ar-implanted AlN/sapphire (AIAS) substrates exhibited selective growth and subsequent lateral growth due to the difference of lattice constants between the implanted and implantation-free regions. Consequently, air voids over the implanted regions were formed around the GaN/AlN/sapphire interfaces. We proposed the growth mechanisms of the GaN layer on the AIAS substrates and characterized the LEDs with embedded air voids. With a 20 mA current injection, experimental results indicate that the light output power of LEDs grown on the AIAS substrates was enhanced by 25% compared with those of conventional LEDs. This enhancement can be attributed to the light scattering at the GaN/air void interfaces to increase the light extraction efficiency of the LEDs.

Improved Output Power of GaN-based Blue LEDs by Forming Air Voids on Ar-Implanted Sapphire Substrate

This paper investigates GaN-based blue light-emitting diodes (LEDs) grown on sapphire substrates with selective-area Ar-ion implantation. The GaN-based epitaxial layers grown on the Ar-implanted sapphire substrates (Ar-ISS) exhibited selective growth and subsequent lateral growth because of different lattice constants between the implantation and implantation-free regions. As a result, air voids were formed at the GaN/sapphire interface, above the implanted regions and below the active layers of LEDs. We proposed the GaN layer growth mechanisms on the Ar-ISS, and characterized the LEDs with embedded air voids at the GaN/sapphire interface. Using a 20-mA current injection, the light output of the experimental LEDs was found to be 15% greater than that of conventional LEDs. This enhancement can be attributed to the light scattering at the textured GaN/air void interfaces, which increases the probability of photons escaping from the LEDs.

Improved conversion efficiency of GaN-based solar cells with Mn-doped absorption layer

GaN-based solar cells with Mn-doped absorption layer grown by metal-organic vapor-phase epitaxy were investigated. The transmittance spectrum and the spectral response showed the presence of an Mn-related band absorption property. Power-dependent, dual-light excitation, and lock-in amplifier techniques were performed to confirm if the two-photon absorption process occurred in the solar cells with Mn-doped GaN absorption layer. Although a slight decrease in an open circuit voltage was observed, a prominent increase in the short circuit current density resulted in a significant enhancement of the overall conversion efficiency. Under one-sun air mass 1.5 G standard testing condition, the conversion efficiency of Mn-doped solar cells can be enhanced by a magnitude of 5 times compared with the cells without Mn-doped absorption layer.

Selective Growth of AlGaN-Based p-i-n UV Photodiodes Structures

Al0.25Ga0.75N ultraviolet (UV) p-i-n photodiodes (PDs) with top and inverted p-layer structure are created by selective-area growth on a GaN template layer. All PDs exhibit typical zero bias peak responsivity at 310 nm and the UV-to-visible (310/400 nm) spectral rejection ratio is greater than three orders of magnitude. In contrast to conventional AlGaN-based p-i-n photodiodes, the inverted devices with low-resistivity n-type AlGaN top-contact layers exhibit a nearly flat spectral response at the short wavelength regions (210-310 nm). The proposed device structure can achieve solar-blind, AlGaN/GaN-based p-i-n PDs with high-aluminum content and reduced tensile strain due to the lattice mismatch between AlGaN and GaN layers.

InGaN Flip-Chip Light-Emitting Diodes With Embedded Air Voids as Light-Scattering Layer

The performance of GaN-based flip-chip light-emitting diodes (LEDs) with embedded air voids grown on a selective-area Ar-implanted sapphire (SAS) substrate was demonstrated in this letter. The GaN-based epitaxial layers grown on Ar-implanted regions exhibited lower growth rates compared with those grown on implantation-free regions. Accordingly, air voids formed over the implanted regions after merging laterally grown GaN facet fronts. The light-output power of LEDs grown on SAS was greater than that of LEDs grown on implantation-free sapphire substrates. The output power of LEDs grown on SAS was enhanced by 20% at an injection current of 700 mA. The increase in output power was mainly attributed to the scattering of light around the air voids, which increased the probability of photons escaping from the LEDs.

White emission from non-planar InGaN/GaN MQW LEDs grown on GaN template with truncated hexagonal pyramids.

Non-planar InGaN/GaN multiple quantum well (MQW) structures are grown on a GaN template with truncated hexagonal pyramids (THPs) featuring c-plane and r-plane surfaces. The THP array is formed by the regrowth of the GaN layer on a selective-area Si-implanted GaN template. Transmission electron microscopy shows that the InGaN/GaN epitaxial layers regrown on the THPs exhibit different growth rates and indium compositions of the InGaN layer between the c-plane and r-plane surfaces. Consequently, InGaN/GaN MQW light-emitting diodes grown on the GaN THP array emit multiple wavelengths approaching near white light.

Photoresponses of manganese-doped gallium nitride grown by metalorganic vapor-phase epitaxy

The photoresponses of gallium nitride (GaN) doped with manganese (Mn) grown by metalorganic vapor-phase epitaxy were investigated. The transmission spectroscopy obtained from the Mn-doped GaN exhibited three distinct absorption thresholds at approximately 365, 650, and 830 nm, respectively. The below-band-gap absorption peaks were attributed to the fact that the deep Mn-related states mediate the electronic transition between the valence and conduction bands. A below-band-gap spectral response ranging from 400 nm to 1000 nm was also observed from a typical GaN p-i-n photodetector with Mn-doped absorption layer. The significant below-band-gap spectral responses showed that the Mn-doped GaN-based materials have promising applications in intermediate band solar cells.

GaN-based photon-recycling green light-emitting diodes with vertical-conduction structure

A p-i-n structure with near-UV(n-UV) emitting InGaN/GaN multiple quantum well(MQW) structure stacked on a green unipolar InGaN/GaN MQW was epitaxially grown at the same sapphire substrate. Photon recycling green light-emitting diodes(LEDs) with vertical-conduction feature on silicon substrates were then fabricated by wafer bonding and laser lift-off techniques. The green InGaN/GaN QWs were pumped with n-UV light to reemit low-energy photons when the LEDs were electrically driven with a forward current. Efficiency droop is potentially insignificant compared with the direct green LEDs due to the increase of effective volume of active layer in the optically pumped green LEDs, i.e., light emitting no longer limited in the QWs nearest to the p-type region to cause severe Auger recombination and carrier overflow losses.

Dual-wavelength GaN-based LEDs grown on truncated hexagonal pyramids formed by selective-area regrowth on Si-implanted GaN templates.

GaN-based blue light-emitting diodes (LEDs) with micro truncated hexagonal pyramid (THP) array were grown on selective-area Si-implanted GaN (SIG) templates. The GaN epitaxial layer regrown on the SIG templates exhibited selective growth and subsequent lateral growth to form the THP array. The observed selective-area growth was attributed to the different crystal structures between the Si-implanted and implantation-free regions. Consequently, LEDs grown on the GaN THP array emitted broad electroluminescence spectra with multiple peaks. Spatially resolved cathodoluminescence revealed that the broad spectra originated from different areas within each THP. Transmission electron microscopy showed the GaN-based epitaxial layers, including InGaN/GaN multi-quantum wells regrown at different growth rates (or with different In content in the InGaN wells) between the semi-polar and c-face planes of each THP.